I've been curious.

[Ashleg]

I apologize if a thread like this has already been made. 
Did Brian dislike any of the animals he used as vermin and like the woodlander species better in general?
Like, what inspired how he decided who would be good and who would be bad? For some of them it's obvious, but others I'm not too sure.
Otters and weasels are pretty similar if you ask me.

[Jetthebinturong]

I assume he chose animals with connotations of evil as mice, squirrels and shrews are also all considered vermin. Otters, badgers and hares are not.

[Skyblade]

That's a good and interesting question, Scartooth! I don't think it's been asked before here, or at least I have never heard of it.

I agree with Jet. It was probably because of the connotations of the species. After all, this is a children and young person's series in general, and Brian wanted to make things simple, clear, and straightforward. The evident distinction between the good and evil species is one of the ways he made everything easier.

For example, take the following vermin species:

Rats - definitely have a negative connotation. Poor things are considered the bigger, evil versions of mice. The fact that they spread diseases doesn't help either.
Weasels - they seem pretty sneaky
Stoats - a lot like weasels...right? Also, even the name sounds like it's one of the bad guy species
Ferret - this one's more surprising, since ferrets are actually good house pets. I guess because they are similar to weasels and stoats.
Foxes - known for being a sly, mysterious, and tricky animal
Lizards - not very adored, sadly
Ravens, crows, etc. - not very well liked species of bird
Snakes - this one's obvious

But as for mice, they seem gentle and harmless enough for a good species. And who doesn't lile squirrels and otters? Hedgehogs and miles just seem benign and friendly. I'm not sure about badgers - maybe they're here to make good beasts more powerful.

[Mhera]

From Ask Brian:

24. Why do you make mice, squirrels, otters and badgers "good" and foxes, rats, ferrets and such "bad"? How do you decide which are good and which are bad? (Donna Wilson, Seattle, Washington)

The bad creatures are those which are traditionally bad in European folk lore and have come to be regarded as sly or mean or evil.The good creatures are mostly small and defenceless, with the exception of the badgers.

[Izeroth]

 Almost all of the vermin are carnivores, and all of the woodlanders are at least partly herbivores. This makes sense: carnivores are naturally more aggressive than herbivores, and also tend not to form peaceful groups.

[Ashleg]

Wow, that makes much more sense!
Thanks. 

[James Gryphon]

Well, otters are technically carnivorous animals, albeit ones that prefer fish and sea creatures, and not too distantly related to ordinary weasels. Badgers will eat just about anything, including hedgehogs (which have become rare in some places, due to badgers), mice, moles, rabbits, and shrews. Intriguingly, they are also capable of tolerating and coexist with foxes. Actually, most of the smaller creatures (including squirrels) have been known to eat other mammals, so I'm not sure how much mileage the "natural diet" view should get.

In addition to the 'folklore' explanation, it's possible, as another person who posted on this subject a while ago said, that Mr. Jacques was influenced by The Wind in the Willows, where Badger is portrayed as an authoritative figure not unlike the Redwall series' Badger Lords. Of course, that book clearly isn't the end-all explanation for everything; after all "Ratty" was a great hero.